Sue, I would say Calmark covers have the best reputation. As for waterproof, you don't really want that. A nice heavy duty fabric cover protects the trailer, and dries out when it gets wet. A watertight cover is not recommended, mainly for mold and mildew reasons.

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"You can't buy happiness, but you can buy an RV. And that is pretty close."

If you don't have wind to contend with you can go with a much cheaper cover. Ours (for our Escape 21)
cost about $200 (Canadian) and looks great after 2 long winters use in the PNW. I expect to get at least a couple more years from it.
We got it on EBay, but Adco makes similar ones. If we had more money to burn I would have gone with a Calmark, because the fit would be better. But if you don't have wind you don't have to worry about the Tyvek material in cheaper covers scuffing the gelcoat finish.